gio: Fix annotations on g_[async_]initable_new() and friends

These return a GObject even though in C it's represented as a gpointer

https://bugzilla.gnome.org/show_bug.cgi?id=693694

gio/gasyncinitable.c

@@ -441,8 +441,8 @@ g_async_initable_new_valist_async (GType                object_type,
  * Finishes the async construction for the various g_async_initable_new
  * calls, returning the created object or %NULL on error.
  *
- * Returns: (transfer full): a newly created #GObject, or %NULL on error.
- *     Free with g_object_unref().
+ * Returns: (type GObject.Object) (transfer full): a newly created #GObject,
+ *      or %NULL on error. Free with g_object_unref().
  *
  * Since: 2.22
  */

gio/ginitable.c

@@ -130,7 +130,8 @@ g_initable_init (GInitable     *initable,
  * similar to g_object_new() but also initializes the object
  * and returns %NULL, setting an error on failure.
  *
- * Return value: (transfer full): a newly allocated #GObject, or %NULL on error
+ * Return value: (type GObject.Object) (transfer full): a newly allocated
+ *      #GObject, or %NULL on error
  *
  * Since: 2.22
  */
@@ -166,7 +167,8 @@ g_initable_new (GType          object_type,
  * similar to g_object_newv() but also initializes the object
  * and returns %NULL, setting an error on failure.
  *
- * Return value: (transfer full): a newly allocated #GObject, or %NULL on error
+ * Return value: (type GObject.Object) (transfer full): a newly allocated
+ *      #GObject, or %NULL on error
  *
  * Since: 2.22
  */
@@ -206,7 +208,8 @@ g_initable_newv (GType          object_type,
  * similar to g_object_new_valist() but also initializes the object
  * and returns %NULL, setting an error on failure.
  *
- * Return value: (transfer full): a newly allocated #GObject, or %NULL on error
+ * Return value: (type GObject.Object) (transfer full): a newly allocated
+ *      #GObject, or %NULL on error
  *
  * Since: 2.22
  */